1. Field of the Invention
The invention relates in general to a copper gate electrode of liquid crystal display device and method of fabricating the same, and more particularly to the copper gate electrode having a barrier layer and the method of fabricating the same.
2. Description of the Related Art
The thin film transistor liquid crystal displays (“TFT-LCD”), having the TFTs arranged in an array and the electrical components (i.e. capacitors, drivers), are capable of displaying the vivid images. With the advantages of handy size, light weight, low power consumption and no radiation contamination, the TFT-LCDs have been widely used in the world. In the commercial market, the TFT-LCD applications include the portable products such as personal digital assistants (PDA), regular size products such as monitors of laptop or desktop computers, and large size products such as 30″˜40″ LCD-TVs.
Conventionally, the gate electrode of the TFT-LCD is made of aluminum alloy. However, the material with higher conductivity is required for the larger-size and high-resolution TFT-LCD, to minimize the wire RC delay. The materials commonly used as the conductive wire include copper (Cu, electric resistance 1.7×10−6 Ωcm), aluminum (Al, electric resistance 2.6×10−6 Ωcm), titanium (Ti, electric resistance 41.6×10−6 Ωcm), Molybdenum (Mo, electric resistance 5.7×10−6 Ωcm), chromium (Cr, electric resistance 12.8×10−6 Ωcm) and nickel (Ni, electric resistance 6.8×10−6 Ωcm). Thus, aluminum alloy replaced by copper has been developed in the recent years.
FIG. 1 illustrates a cross-sectional view of a partial structure of a conventional TFT-LCD. A copper layer is sputtered on a transparent glass substrate 101, and the copper layer is etched to form a patterned copper layer (i.e. as the gate electrode of the TFT-LCD) 103 by photolithography. It is a need for the patterned copper layer 103 to have the appropriate taper angles in the sidewalls. Afterward, a silicon nitrite layer 105, an a-Si (amorphous silicon) layer 107 and a n+ a-Si layer 109 are laminated above the patterned copper layer 103.
Although copper possesses a good conductivity, the conventional process of fabricating the conductive wires (i.e. gate electrode) using copper still has several problems to be solved. For example, surface oxidization quickly occurs and it is not easy to control the taper angle of the patterned copper layer due to the difficulty of copper etch. The adhesion strength between the patterned copper layer 103 and the glass substrate 101 is weak, so is the adhesion between the patterned copper layer 103 and the silicon nitrite layer 105. If the patterned copper layer 103 directly contacts with the silicon nitrite layer 105, copper quickly reacts with silicon to produce Cu3Si so as to change the electrical properties of the applied device (i.e. TFT-LCD). Also, copper diffused into the silicon nitrite layer 105 deteriorates the insulation property of silicon nitrite so as to increase the current leakage. Moreover, the bare patterned copper layer is reactive in the sequential process such as plasma enhanced chemical vapor deposition (PECVD) or dry etching process; thus, it is easy to contaminate the processing machine so as to degrade the quality of the applied device.
Some attempts have been made for solving the problems listed above. The first attempt is to dispose at least one metal layer between the patterned copper layer 103 and the silicon nitrite layer 105 to solve the problems of weak adhesion, reactivity and diffusion between copper and silicon. The metal layer could be made of tantalum nitride (TaN)-titanium nitride (TiN)-aluminum nitride (AlN)-aluminum oxide (Al2O3)-titanium oxide (TiO2)-tantalum (Ta)-molybdenum (Mo)-chromium (Cr)-titanium (Ti)-tungsten (W) and nickel (Ni). However, additional steps such as deposition, developing and etching are required for forming this metal layer. The second attempt is to use the copper alloy such as an alloy of copper and chromium (Cu1-xCrx), or an alloy of copper and magnesium (Cu1-xMgx) as the material of the patterned copper layer 103. Also, the thermal oxidation is applied to form chromium oxide (Cr2O3) or magnesium oxide (MgO) on the surface of the patterned copper layer 103 for solving the problems of weak adhesion, reactivity and diffusion between copper and silicon. Similarly, the second attempt requires extra steps such as metal deposition, developing, etching and thermal oxidation during the fabrication. The third attempt is to dispose an indium tin oxide (ITO) layer between the patterned copper layer 103 and the transparent glass substrate 101 for solving the problem of weak adhesion between the copper and glass.
Moreover, the improper taper angle of patterned copper layer causes the impact of film coverage of post processes, and therefore the yield of production is decreased. The three attempts discussed above cannot control the taper angle of patterned copper layer; a need still exists for a method of obtaining a proper taper angle.